Fast neutron radiography (FNR) is useful in interrogating bulky objects for which alternative probes, such as slow neutrons or low-E X-rays, are limited in penetration. Due to the weak dependence of fast neutron cross-sections on atomic number Z, the transmission attenuation in the interrogated object is predominantly determined by its mean atomic density. A notable exception prevails at neutron energies <˜2 MeV, where hydrogen is the dominant attenuator, if present in appreciable quantities.
The possibility of utilizing characteristic resonances in fast-neutron cross-sections has led to the development of a multi-element-specific fast-neutron radiography and tomography method (FNRT) based on pulsed-beam TOF measurements. (See Overley J. C., “Determination of H,C,N,O Content of Bulk Materials from Neutron Attenuation Measurements”, Int. J. Appl. Rad. & Isot. 36 (1985) 185, and Lanza R. C. et al., Illicit Substance Detection, Gordon Research Conf., Conn. College, New London Conn. (July 2000).) It exploits modifications of broad-energy neutron spectra transmitted through an inspected object, due to characteristic cross-section energy-variations of elements present.
With the FNRT method, the object is illuminated by a pulsed broad energy (0.8-4 MeV) neutron beam produced by short repetitive beam bursts of 4-5 MeV deuterons impinging on a thick Be target. Time of flight is used to measure the energy dependence of neutron transmission through an object. The method has been applied to detecting elements such as C, O, N & H for determining the composition of agricultural products and for the detection of contraband. (See Fink C. L., Micklich B. J., Yule T. J., Humm P., Sagalovsky L. and Martin M. M., Nucl. Instr. & Meth. B99 (1995) 748, and Overley J. C., Chmelnik M. S., Rasmussen R. J., Schofield R. M. S. and Lefevre H. W., Nucl. Instr. & Meth. B99 (1995) 728.) A system for detection of explosives in air passenger bags based on this method has also been constructed and tested. (See Overley J. C., Chmelnik M. S., Rasmussen R. J, Sieger G. E., Schofield R. M. S. and Lefevre H. W., SPIE, Vol. 2867 (1997) 219; Miller T. G., Van Staagen P. K. and Gibson B. C., SPIE, Vol. 2867 (1997) 215; and Van Staagen P. K., Miller T. G. and Gibson B. C. and Krauss R. A., Proc. of 2nd Explosives Detection Technology Symp. and Aviation Security Technol. Conf., Nov. 12-15, 1996.)
On the instrumental side, fast-neutron-based imaging techniques are, in general, harder to implement than those for thermal neutrons, primarily due to the fact that typical fast-neutron detection efficiencies are in the 0.01% to 20% range, compared to 20-70% for thermal neutrons.
Neutron detectors employed in FNR are mostly based on the following devices:
1. Scintillating screens viewed by CCD
2. Plastic scintillator slabs viewed by CCD
3. Scintillating fiber screens viewed by CCD
4. Hydrogenous or metallic converter foils coupled to charged-particle detectors
The above are now discussed in greater detail.
1. Scintillating screens are based on hydrogen-rich—materials such as polypropylene loaded with ZnS(Ag) scintillator. Knock-on protons interact with the scintillator and the emitted light is detected with a CCD camera via an appropriate optical system. (See Yoshii K. and Miya K., Nucl. Instr. & Meth. A346 (1994) 253 and Ambrosi R. M. and Watterson J. I. W., Nucl. Instr. & Meth. B139 (1998) 279.) Light outputs of ˜500 photons per incident 1 MeV neutron have been reported. (See Brenizer J. S., Berger H., Gibbs K. M., Mengers P., Stebbings C. T., Polansky D. and Rogerson D. J., Nucl. Instr. & Meth. A424 (1999) 9.) Large screens of up to 30×30 cm2 can be constructed using this technique. The minimum detectable neutron flux is determined by the optical geometry and CCD noise. In most cases a cooled CCD camera is used. An undesirable feature of these detectors is that they tend to have non-negligible efficiencies for gamma-rays. Moreover, ZnS, being a slow scintillator, is unsuitable for fast timing applications.
2. A plastic scintillator slab coupled to a CCD camera is another version of the above detector. A 4 cm thick slab with an active area of 30×30 cm2 was developed. (See Hall J. M., Neutron Tomography: Illicit Substance Detection, Gordon Research Conf., Conn. College, New London Conn., July 2000.) Due to its large thickness the detector has relatively high detection efficiency, but its spatial resolution is poor (2-3 mm). It too suffers from high sensitivity to gamma-rays.
3. The scintillating fiber screen consists of a bundle of solid or liquid-core scintillating fibers coupled to a CCD readout. Position resolution depends on fiber diameter and length. Small diameter ensures good spatial resolution, but this is at the expense of higher cross-talk and reduced light transmission. The length of the fiber determines detection efficiency and penumbra effects. (See Brzosko J. S., Robouch B. V., Ingrosso L., Bortolotti A. and Nardi V., Nucl. Instr. & Meth. B72 (1992) 119, and Holslin D., Armstrong A. W., Hagan W., Shreve D. and Smith S., Nucl. Instr. & Meth. A353 (1994) 118.) The minimum detectable neutron flux is determined by the optical geometry and CCD noise. In most cases a cooled CCD camera is used.
4a. A hydrogenous converter foil detector consists of a hydrogenous radiator coupled to a position-sensitive charged-particle detector (solid-state or gas), which detects the knock-on protons. (See Hosono Y. et al., Nucl. Instr. & Meth. A361 (1995) 554.) The spatial resolution of the detector is determined by the length of proton trajectories in the gas and by the resolution of the position-sensitive readout.
4b. A metallic converter foil detector consists of a foil in which the neutron interacts primarily via the (n,p) reaction. The resulting proton is registered in a position-sensitive detector. By choosing a reaction with a given threshold energy, one can reject scattered neutrons with energies below threshold. Both multi-wire chambers and micro-strip gas detectors have been developed. (See Bertalot L., Bencivenni G., Esposito B. and Pizzicaroli G., Nucl. Instr. & Meth. A409 (1998) 20, and Morris C. L., Armijo V., Atencio L. G., Bridge A., Gavron A., Hart G., Morley K., Mottershead T., Yates G. J. and Zumbro J., Proc. Int. Conf. On Neutrons in Research and Industry, Crete, Grece, (1996), 351.) However, since (n,p) cross-sections are typically below 500 mb and the foils must be thin enough for protons to emerge into the gas with appreciable energy, detection efficiencies are correspondingly low (see Table I below).
Table I compares characteristics of FNR detectors developed over the last decade.
TABLE ICharacteristics of contemporary FNR detectorsScintillatingSlab plasticScintillatingMetallic converter/PropertyScreen/CCDscintillator/CCDFibers/CCDgas detectorSpatial Resolution250-20002000-3000500 (depending400 (dependingFWHM [μm]on fiber dim's.)on readout)Efficiency (per1%20%4-6%0.2-0.6%incident fast-n)(2 mm thick)(40 mm thick)(100 mm long)(200-2000 μm thick)Gamma sensitivityYesYesYesNoTimingNoNoNo10 nsDetector area30 × 30 (limited30 × 3010 × 1012 × 12realized [cm2]by optics, CCD
From Table I it is evident that most contemporary fast-neutron imaging detectors suffer from low detection efficiency and lack of timing capability. The slab plastic scintillator has relatively high efficiency, but this advantage is offset by its poor spatial resolution. The correlation between these two parameters can be reduced by the use of scintillating fibers, for which the spatial resolution is dependent on fiber diameter and the knock-on proton range in the fiber. Another characteristic of detectors based on scintillators is their sensitivity to gamma-rays. The metallic converter detector counts single events and is therefore able to provide information on neutron energy by measuring its time of flight. Although insensitive to gamma-rays, its detection efficiency is extremely low.
Requirements on imaging detectors for FNRT methods are more stringent than for FNR due to the fact that neutron spectrometry with rather good (typically, 100-500 keV) resolution is a prerequisite for the technique. The requirements from such detectors are:
1. large sensitive area
2. position resolution of <1 mm
3. efficient detection of fast-neutrons over a broad energy range
4. neutron spectroscopy capability within this energy range
5. insensitivity to gamma-rays
6. ability to operate at high counting rates
High-speed arrays of detectors for contraband identification using FNRT have been proposed and developed (See Van Staagen P. K., Miller T. G., Gibson B. C. and Krauss R. A., Proc. of 2nd Explosives Detection Technology Symp. and Aviation Security Technology Conf., Nov. 12-15, 1996 and Gibson B. C., Miller T. G., Van Staagen P. K. and Krauss R. A., Proc. of 14th Int. Conf. on Applications of Accelerators in Research & Industry, Nov. 6-9, 1996.) They consist of a matrix of individual scintillation detectors positioned as a 2-dim. array. Each detector is coupled to a light-guide, photomultiplier and electronics. Pixel dimensions achieved with these arrays are in the few-cm range.
Another arrangement for an x-y FNRT neutron detector was proposed (See Miller T. G., “High Energy X-Y Neutron Detector and Radiographic/Tomographic Device”, U.S. Pat. No. 5,410,156, 1995.) The detector consists of a stack of separate, scintillating fiber bundles, which form a plane. One coordinate is determined by the bundle struck by the neutron. The other coordinate obtains by measuring the time difference of scintillation photons in reaching opposite ends of the fiber-optic strand. Position resolution obtained was of the order of 4×4 cm2.
The relatively poor position resolution obtained in the above-mentioned devices did not permit reliable detection of small and thin objects. (See “The Practicality of PFNTS for Aviation Security”, NAS Panel report, 1999, http://books.nap.edu/html/aviation_spectroscopy/.)
To appreciate the influence of detector properties on performance characteristics of an FNRT inspection system, it is instructive to consider the basics of the time of flight (TOF) method for measuring neutron energy. In TOF, an accelerator ion-beam is pulsed to generate a short (1-2 ns) neutron burst via a nuclear reaction. In the non-relativistic limit, which is valid for En<˜10 MeV, the time required by individual neutrons in a pulse to reach a neutron detector positioned at a fixed distance from the source can be measured and converted to neutron energy via the simple relation:
      E    n    =            1      2        ⁢                  m        ⁡                  (                      d                          t                              T                ⁢                                                                  ⁢                O                ⁢                                                                  ⁢                F                                              )                    2      where d is the source-detector distance, m the neutron mass and tTOF the time-of-flight.
The overall time resolution (a convolution of the duration of the beam burst and the instrumental time resolution of the detector) thus determines the energy-bin size. For example, a TOF distance of 5 meters and overall time resolution of 5 ns translates to an energy-bin size of 0.3 MeV at EN=5 MeV. Most fast-neutron resonances are considerably narrower and can thus not be resolved; however, certain cross-sections of interesting elements such as C, N, O do fluctuate over energy-intervals that correspond to the TOF resolving power of the radiography system. In such cases, the contrast sensitivity for element-specific FNRT will depend predominantly on the instrumental time resolution of the detector, since typical accelerator beam bursts are short (1-2 ns). Thus, the goal for operational FNRT detectors is a timing resolution of ˜2 ns. The current figure-of-merit for the present invention, TRION, is ˜10 ns, with good prospects for reducing this value to ˜5 ns in the near future and possibly even better, eventually.
Single-event-counting (SEC) is the conventional, most widely used TOF mode. Here, it is essential that the probability for a neutron to be detected in a single accelerator beam-burst be low (<˜10%). The reason is that, if more than one neutron is detected per burst, only the first will be counted, resulting in pile-up counting losses and spectrum distortion. This restriction severely limits detector counting rates and does not permit operation at high neutron flux intensities. The effect can be countered by a high degree of segmentation of the data-acquisition system, but such a solution is costly.
SEC data acquisition is usually in list mode, the relevant parameters such as position and TOF of each individual event being measured and stored in a multi-dimensional histogram. The advantage of this method is that very good parameter definition can be achieved, usually in subsequent off-line analysis. This is, however, at the expense of considerably reduced operating speed as well as increased data file size.